Preheat the oven to 350 degrees F and lightly grease an 8-cup muffin tin.
In a large bowl, combine the ground beef, onion, green bell pepper, egg, milk, crushed saltine crackers, salt, and black pepper. Mix until evenly blended.
Divide the mixture evenly among the 8 muffin cups and press down lightly to fill each cup.
In a small bowl, stir together the ketchup, brown sugar, and yellow mustard until smooth.
Spoon the glaze over the top of each meatloaf muffin, spreading it to cover the surface.
Bake for 25 to 30 minutes until the internal temperature reaches 160 degrees F and the glaze has thickened.